As a conventional device redundancy technique in the computer network, for example, as disclosed in Non Patent Literature 1 (VSS: virtual switching system) and Non Patent Literature 2 (vPC: virtual port channel), there has been known a system in which a dedicated line for allowing a data plane traffic and a control plane traffic crossing between devices to pass therethrough is prepared, and traffic is crossing between the control planes on the basis of a protocol to virtually realize device redundancy. Both techniques of Non Patent Literature 1 and Non Patent Literature 2 are to perform a link-aggregation (LA) between two devices to realize the device redundancy. A difference between the respective literatures resides in that the number of control planes (viewed from the network) appears to be one (Non Patent Literature 1) or two (Non Patent Literature 2).
With progress of an information and communication technology (ICT), a router having a high processing performance has been demanded. As one method for providing the router having the high processing performance, there is virtualization of the router. The virtualization of the router means that a plurality of routers are bundled together, and functions as one virtual router. The virtualized router has not only the high processing performance, but also can continue processing by the other routers except for a router that is in failure, for example, even if a part of routers (for example, one of three routers) is in failure, resulting in an advantage that the redundancy of the network system can be realized.
As an example of a method for realizing the virtualization of the router as described above, there has been known, for example, a method in which a plurality of routers are connected to each other through a device only for switching, and acts as one virtual router to provide the router having the high processing performance (for example, refer to Non Patent Literature 3). Also, as another example of the method for realizing the virtualization of the router, there has been known, for example, a method in which external ports (ports for connection to external terminals) of a plurality of routers are connected to each other, for example, with the use of a LAN cable, and act as one virtual router to provide the router having the high processing performance.